Origin is not in the horizontal middle for a model which I "overlaid"
Further to @cegaton's answer.. the behavior of Shift S Cursor to Selected respects the current Pivot point.



in Edit Mode with all selected:


if the Pivot is set to 'Median', the cursor goes to the location of
the smaller empty in the illustration, at an average position of of
Suzanne's selected vertices, giving them all equal weight. Since she
has more vertices in her face than at the back of her head, that
drags the location towards the front.
If the Pivot is set to 'Bounding Box' the cursor is sent to location
of the larger empty, at the center of Suzanne's bounding box.


I'm not sure if this is what you want, but illustrated below is the result of Shift S Cursor to Selected in Edit Mode, with all vertices selected, and the Pivot set to Bounding Box Center, followed by  Shift Alt O in Object Mode > Origin to 3D Cursor



The cursor and origin are at the center of the object's bounding box.

The "Bounding Box Center" option is located here: